| 18 | |
| 19 | // Returns an observable which outputs an AST from an input observable of token objects |
| 20 | function parse(tokenStream) { |
| 21 | |
| 22 | let current = null; |
| 23 | |
| 24 | function* peek() { |
| 25 | |
| 26 | if (current === null) |
| 27 | current = yield; |
| 28 | |
| 29 | return current; |
| 30 | } |
| 31 | |
| 32 | function* eat(type = "") { |
| 33 | |
| 34 | let token = yield * peek(); |
| 35 | |
| 36 | if (type && token.type !== type) |
| 37 | throw new SyntaxError("Expected " + type); |
| 38 | |
| 39 | current = null; |
| 40 | return token; |
| 41 | } |
| 42 | |
| 43 | function* parseAdd() { |
| 44 | |
| 45 | let node = yield * parseMultiply(); |
| 46 | |
| 47 | while ((yield * peek()).type === "+") { |
| 48 | |
| 49 | yield * eat(); |
| 50 | let right = yield * parseMultiply() |
| 51 | node = { type: "+", left: node, right, value: node.value + right.value }; |
| 52 | } |
| 53 | |
| 54 | return node; |
| 55 | } |
| 56 | |
| 57 | function* parseMultiply() { |
| 58 | |
| 59 | let node = yield * eat("NUMBER"); |
| 60 | |
| 61 | while ((yield * peek()).type === "*") { |
| 62 | |
| 63 | yield * eat(); |
| 64 | let right = yield * eat("NUMBER"); |
| 65 | node = { type: "*", left: node, right, value: node.value * right.value }; |
| 66 | } |
| 67 | |
| 68 | return node; |
| 69 | } |
| 70 | |
| 71 | function* start() { |
| 72 | |
| 73 | let ast = yield * parseAdd(); |
| 74 | yield * eat("EOF"); |
| 75 | return ast; |
| 76 | }; |
| 77 | |